This invention relates generally to electronic tuning systems for television receivers and methods therefore, and more particularly relates to an indirect frequency synthesis television tuning system and method in which automatic frequency control is provided over the entire automatic fine tuning range.
The use of indirect frequency synthesis in television tuning systems is increasing because of the inherent advantages of this frequency tuning approach. The indirect frequency synthesis tuning system utilizes a phase lock loop (PLL) that employs the local oscillator (LO, of the tuner for its voltage controlled oscillator (VCO). A stable, accurate reference signal is generated by a crystal oscillator and compared to the output of the LO that has been divided down in frequency in the PLL. By counting down from the LO which provides one input to the phase comparator, the PLL causes the signal thus produced to be exactly the same frequency as the reference signal which provides the other input to the phase comparator. The two signals will be slightly out of phase due to the static phase error of the PLL. With the PLL forcing both of the inputs to the phase comparator to be equal in frequency, a change in the division ratio between the LO and the phase comparator will result in a different LO frequency.
With the availability of electronic television tuning systems employing voltage synthesis, extremely accurate tuning voltages may be thus generated but tuner drifts will result in the same tuner producing different LO frequencies at different times even though exactly the same tuning voltage is applied. As a result of these inherent inaccuracies in a voltage synthesis system, automatic fine tuning (AFT) circuitry is generally provided in combination with a frequency discriminator. The frequency discriminator is characterized by an AFC S-tuning curve whereby the tuned-to frequency is determined by the input synthesized tuning voltage. This S-curve determines the system's frequency as a function of applied tuning voltage and, because of its S-shape, can result in tuning to false signals unless additional signal detection and verification means are provided. Without these additional signal detection and verification means, for example, several signals of different frequency may be tuned to by a television tuning system using the same tuning voltage input.
The present invention is intended to avoid these signal tuning problems by providing an indirect frequency synthesis system which is controlled by a microcomputer and which offers automatic frequency control over the entire automatic fine tuning range of the tuner.